Behavior modification associated with agonistic experience recently has been observed in Drosophila melanogaster. Specifically, Drosophila males exhibit a "loser effect": if a male loses his first dyadic competition, he will never win a subsequent fight when paired with a naive male with no previous fighting experience, 30 minutes after the initial contest. In addition, subordinate males use different fighting strategies depending on if they are paired with a new or previous opponent, suggesting that fruit flies are capable of recognizing individuals. While behavioral changes following agonistic experiences have been documented for numerous animals, little is known about the genes and neural circuits that regulate these changes. A number of genes have been identified through olfactory conditioning assays that are necessary for learning and memory in Drosophila. It can be addressed whether these same genes function in operant learning associated with aggression by testing whether the classical learning and memory mutants exhibit behavioral changes following agonistic experiences. Similarly, behavioral assays can be used to identify neural circuits that are required for learning and memory associated with aggression, since neuronal Gal4 lines can drive expression of UAS-shibirets1 to block synaptic transmission and effectively turn off subsets of neural circuits. It is likely that some subsets of neural circuits that function in opponent recognition are not responsible for loser effects. Sensory systems such as olfactory and gustatory systems that detect cuticular hydrocarbons and pheromones could possibly be used for recognizing individuals. This can be tested by pairing appropriate sensory mutants in fighting assays. Finally, behavioral assays with longer recovery periods between first and second fighting contests can be used to determine if Drosophila generates long- term memory associated with losing experience. Symptoms of aggression are often present in neurological disorders such as Alzheimer's disease, intermittent explosive disorder and other mood disorders, yet the physiological control of aggression in healthy and ill patients is still very vague. In addition, it is anticipated that both genes and experience contribute to the onset of psychiatric conditions. Understanding how experience brings about neuronal and behavioral modifications in the Drosophila model of aggression, an innate behavior in fruit flies, could lead to an understanding of how human psychiatric disorders develop, as well as aid in their treatment and prevention.